feat(11-01): create sites table migration, model, and schemas

- Add migration 030 with sites table, RLS policy, and device site_id FK
- Add Site SQLAlchemy model with tenant isolation
- Add site_id nullable FK and relationship to Device model
- Add sites relationship to Tenant model
- Register Site in models __init__.py
- Add SiteCreate, SiteUpdate, SiteResponse, SiteListResponse schemas

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/models/device.py

@@ -101,6 +101,13 @@ class Device(Base):
     tag_assignments: Mapped[list["DeviceTagAssignment"]] = relationship(
         "DeviceTagAssignment", back_populates="device", cascade="all, delete-orphan"
     )
+    site_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("sites.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+    site: Mapped["Site"] = relationship("Site", back_populates="devices")  # type: ignore[name-defined]
 
     def __repr__(self) -> str:
         return f"<Device id={self.id} hostname={self.hostname!r} tenant_id={self.tenant_id}>"